<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style type="text/css">
    .box {
        width: 200px;
        height: 200px;
        border: 1px solid red;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    .children-box {
        background: yellow;
        height: 100px;
        width: 100px;
    }
    </style>
</head>

<body>
  <div id="app">
    <div class="box">
      <div class="children-box"></div>
    </div>
  </div>
</body>

</html>